The present invention relates to furnaces and particularly to improved apparatus for regulating air flow through a port introducing combustion air into the firebox of a furnace.
Wood pulp for papermaking is usually manufactured according to the sulfate process wherein wood chips are treated with a cooking liquor including sodium sulfide and sodium hydroxide. The wood chips and the cooking liquor, called "white liquor", are cooked in a digester under predetermined heat and temperature conditions. After cooking, the used liquor, termed "black liquor", containing spent cooking chemicals and soluble residue from the cook, is washed out of the pulp and treated in a recovery unit where the cooking chemicals are reclaimed. Without reclamation and reuse of the cooking chemicals, the cost of the papermaking process would be prohibitive.
In the recovery process, the black liquor is first concentrated by evaporation to a water solution containing about 65 percent solids, which solution is then sprayed into the firebox of a black liquor recovery boiler, a type of chemical reduction furnace. The chemical reduction furnace is a reactor wherein the processes of evaporation, gasification, pyrolysis, oxidation and reduction all occur interdependently during recovery of the cooking chemicals. The organic materials in the black liquor, lignin and other wood extracts, maintain combustion in the firebox, and the heat produced dries and melts the spent cooking chemicals as they fall to the floor of the firebox, where they build a mound of material called a char bed. The char bed is further heated to further liquefy the chemicals into a molten smelt that flows out of the furnace through a smelt spout to a collection tank. Concurrently, combustion heat is employed to generate steam in a water wall of the boiler for use as process steam and for generating electricity.
The combustion process requires the introduction of large volumes of air into the firebox, air comprising about 80 percent of the material entering the furnace. The combustion air is distributed by means of wind boxes or ducts disposed at several levels in surrounding relation to the firebox and outside the walls of the furnace. The air is forced into the firebox from the wind boxes through a plurality of passages or ports in the walls of the furnace, viz.: primary, secondary and tertiary air ports. The primary air ports, through which about 40 to 50 percent of the air enters the furnace, are disposed on the side walls of the firebox near the bottom of the furnace and close to the char bed. The air supplied to the primary air ports is at a comparatively low pressure in order to promote a reducing atmosphere in the burning mass of char. The secondary air ports, which are fewer in number than the primary air ports and through which about 35 percent of the air enters the furnace, are disposed around the walls of the firebox, higher than the primary air ports, and usually below the level of the entry conduits through which the black liquor is sprayed into the firebox. Air supplied through the secondary air ports is at a slightly higher pressure in order to promote burning of combustible gases rising from the glowing mass of the char bed. While the primary air ports provide a relatively large volume of air with considerable turbulence for maintaining a fireball in the char bed, the secondary air ports are intended to provide a finer control and distribution of air above the char bed and distribute the air evenly in the black liquor spray to support the combustion thereof. Air is supplied through the tertiary air ports at a still higher pressure to promote combustion of gases rising through the firebox, the tertiary air ports being higher on the wall of the furnace than the secondary air ports.
The black liquor sprayed into the firebox, having a consistency like warm 60 weight oil, swirls, burns and falls toward the bottom of the firebox in the form of combustion products comprising char material and smelt. The smelt and char material contact and flow down the outer walls of the firebox and, cooled by the inflowing air, form excrescent deposits around edges of the air ports, particularly along the top edges of the secondary air ports where the excrescent material builds up and outward under influence of air rushing through the secondary air port. Such buildup of char material can block air flow through a port causing furnace operation to be inefficient and unpredictable with an attendant decrease in the amount of chemicals that can be recovered, a decrease in the amount of steam produced per unit of fuel, and increased emission of noxious gases such as hydrogen sulfide, carbon monoxide and sulfur dioxide. In accordance with customary practice, the char buildup is periodically removed, for example, by manually inserting a cleaning rod into the air ports successively around the boiler or by actuating mechanized cleaning apparatus mounted in the air ports. With the passage of time, and particularly when cleaning is effected by manual rodding of the air ports, undesirable fluctuations in combustion can occur. Gradual buildup of char material intermittently around the furnace can cause substantial changes in the volume of combustion air, as well as changes in air distribution, velocity and pressure.
The volume and distribution of combustion air supplied to the furnace will also vary depending on the load of the furnace and the moisture content of the liquor being reduced. The distribution and volume of air entering a furnace is conveniently adjusted by regulating means such as dampers provided in supply conduits of the wind boxes. Dampers may also be provided at various locations in the wind boxes, and individual air ports may furthermore be provided with a damper, thus making possible a selective distribution of air within each wind box, or in each wind-box passage or each air port, thereby maintaining the desired air supply in all parts of the furnace.
Viewing and access ports are provided in the wind box adjacent to the air ports of recovery furnaces. Monitoring devices such as pyrometers for sensing temperature inside the firebox and television cameras for viewing conditions inside the furnace are often installed in such access ports. A damper installed in the air port may block the view through the viewing port or interfere with the operation of sensors, and it has been necessary in the past to remove the damper before installing a sensor.
Separate apparatus for cleaning openings in a recovery furnace are known. See, for example, U.S. Pat. Nos. 4,423,533 and 4,822,428 to Byron L. Goodspeed. Apparatus combining the function of airport cleaning and air-flow damping are also known, e.g., see U.S. Pat. Nos. 4,748,004 and 4,838,182 to Byron L. Goodspeed and U.S. Pat. No. 4,846,080 to Ross et al. Dual purpose apparatus such as disclosed in the Goodspeed U.S. Pat. No. 4,748,004 can have some disadvantage in either the air damping function or the cleaning function. According to Goodspeed U.S. Pat. No. 4,748,004, the cage-like structure of the cleaning head is enclosed, and the mounting frame is partially or completely enclosed or walled in, so that, for a given position of the cleaning head, air flow tends to be closed off. The position of the cleaning head may be varied to accomplish cleaning or to effect a different air flow. It has been found that a cleaning element utilized as a damper partially or fully blocking an air port, extending somewhat into the air port, may be subjected to excessive heat from the furnace. Moreover, a device designed to function effectively as a damper, when employed for cleaning the air port, may be less effective for that purpose because the damper shape may not be conducive to cleaning the opening. It is thus often desirable to employ a damper that is separate from the cleaning element and that is disposed near but spaced some distance inside the wind box from the air port.
A damper that controls the air flow to a particular air port is, however, ordinarily located somewhat near the air port; toward the top of the air port opening, and consequently could interfere with the operation of the automatic cleaning apparatus installed in the air port. If space were available, a damper might be installed upstream of the cleaning apparatus in the passage supplying air to the air port; however, space adjacent an air port is limited. When a damper is retracted or moved out of the way of cleaning apparatus, it is sometimes necessary to position the damper such that air flow through the passage to the air port is blocked, but this is undesirable because without positive air pressure outside the air port, effluent from the furnace could back flow into the air duct. A retracted damper that blocks the flow of air to the air port can also block the view of an observer attempting to see the furnace interior through a viewing port associated with the air port opening, or interfere with apparatus such as sensors inserted into the plenum.
Ross et al in U.S. Pat. No. 4,846,080 disclose apparatus that positions the damper in a full open position, in longitudinal orientation with respect to the plenum containing the apparatus, and which then retracts the damper longitudinally away from the air port opening and out of the way of other devices such as cleaning apparatus and sensors. While this mechanism functions efficiently and satisfactorily, the longer length of the plenum required to house the apparatus has been found to be a problem in some installations. Further, when the cleaning head requires service or replacement, it has heretofore been necessary to remove the plenum from the air port.
It is accordingly a primary object of the present invention to provide improved apparatus for regulating the flow of combustion air in a chemical recovery furnace.
It is a more particular object of the present invention to provide improved air regulating apparatus installable in an air port of a chemical recovery furnace and including a damper mechanism that operates cooperatively in a foreshortened plenum with air port cleaning apparatus.
It is another object of the present invention to provide separate air damping and air port cleaning apparatus operating cooperatively inside a plenum, either of which can be removed from the plenum.
Another object of the present invention is to provide improved air regulating apparatus for increasing the operational stability of a black liquor recovery boiler.
It is another object of the present invention to provide improved apparatus installable in an air port of a chemical recovery furnace for regulating air flow through the air port, which apparatus can be retracted without interfering with the operation of a sensing device associated with the air port wherein the air flow regulating apparatus is installed.
Yet another object of the present invention is to provide improved apparatus installable in an air port of a chemical recovery furnace for regulating the flow of combustion air through the air port, said apparatus being retractable without blocking air flow through the air port.
It is a further object of the present invention to provide improved apparatus regulating air flow through an air port of the furnace of a black liquor recovery boiler for enhancing the efficiency of chemical recovery, increasing steam production, and reducing emissions of sulfur dioxide and carbon monoxide.